Fragile X syndrome is the leading inherited cause of intellectual disability, with the majority of males (95%) having IQs below 70. Most males with fragile X display autistic-like behaviors, and 25-30% meet the DSM-IV criteria for an autism co-diagnosis. Males with comorbid fragile X and autism are typically reported to have more severe overall impairments in cognition and receptive/expressive language. However, the language phenotype within fragile X only and comorbid fragile X and autism is extremely variable across individuals. Moreover, the impact of autism on the language phenotype of fragile X is not clear. The purpose of the proposed project is to determine if there is a grammatical deficit in FXS and in autism, while examining the consequences of comorbid autism on language in fragile X. Research on language development in developmental disabilities to date has been descriptive in nature, as opposed to theory-driven. There is however, a rich theory-driven line of research on grammatical development in children with language impairments. The extended optional infinitive account (EOI) posits that children with specific language impairment seem to get stuck in an optional infinitive stage in which they treat the use of certain grammatical morphemes as optional, despite their obligatory status in the adult grammar. Preliminary evidence suggests that children with fragile X and children with autism may demonstrate a deficit in at least one aspect of grammatical development (i.e., finiteness marking; past tense: he walked) independent of nonverbal IQ. This project will advance preliminary work by examining all finiteness markers (BE/DO), as well as non-finiteness grammatical morphemes (i.e., plural -s, possessive -s, present progressive -ing). This investigation will inform whether boys with fragile X show a grammatical profile similar to SLI, or if their language is characterized by global difficulty with all grammatical morphemes. In addition, the impact of co-morbid autism on grammatical development will be examined by comparing boys with fragile X with and without autism and boys with idiopathic autism. Additionally, the proposed study will examine the most effective method of assessing grammar in boys with FXS and boys with autism. Sixty-three boys between the ages of 9-16 years will participate in this study: boys with FXS only, boys with FXS and autism, and boys with idiopathic autism. Participants will complete standardized tests, a language sample, autism diagnostic measures, and a sentence imitation task. These results will inform the phenotypes of fragile X and autism and provide insights into the nature of variability in language impairments across different etiological conditions. The information from the proposed studies will inform (1) hypotheses regarding neural constraints on language development as well as (2) clinical assessment and intervention procedures. This study lays the groundwork for future studies needed to assess additional aspects of the linguistic system.